The present invention relates to a connection polarity switching apparatus which is suitably used in a network that performs communications, using cables having polarities, and in particular, twisted pair cables with shields.
A twisted pair cable is one type of cable which is used for communications and has two mutually insulated wires twisted around each other, and is characterized as having a strong noise resistance since the respective influences of external disturbances upon each of the cables cancel each other out. Because of this, such twisted pair cables are used in environments, such as in vehicles, where there is a great deal of external noise. Furthermore, there is also the advantage that communications can be continued using the other wire should one of the wires be broken. Owing to this high reliability, twisted pair cables are selected for communication lines of computers that perform important control such as that of engine control system and hydraulic systems.
However, there are problems associated with twisted pair cables. The connection work of twisted pair cables, and in particular, for twisted pair cables having shields is very complex.
FIG. 1 shows a system where a plurality of communication control apparatuses 2A, 2B, 2C, 2D are connected to the same cable, and where this cable 1 is used as a shared bus. When a communication control apparatus is newly installed to such a system, the new apparatus must be branch connected with respect to the cable 1, and this work is performed as shown in FIG. 2(A) through (F).
First, as shown in FIG. 2(A), the cable 1 that configures the bus and the cables that are lead out from the communications control apparatus 2, are cut at suitable positions as shown by the broken line. Then, sheaths 3, shield meshes 4 and vinyl outer coverings 5 are successively removed to expose the wires 7 through 9. When these processes been completed, wires of both cables are connected to the connector 10, as shown in FIG. 2(F). Moreover, wires 8 and 9 correspond to one each of the lines 1a and 1b in FIG. 1.
In this manner, the branching and connection work for twisted pair cables involves many processes and is annoying and time consuming.
Furthermore, when there is such a connection structure, the shield effect deteriorates because the shield of the branching and connection portion must be removed, and signal leaks, the entry of external noise, signal reflection and the like due to changes in the line impedance (specifically, capacitance and inductance) cause problems of the generation of waveform distortion of signals to be transmitted.
In order to perform the branching and connection of coaxial cables by extremely simple operations, one conventional method using a needle-shaped connection member is widely used. Connection work using this needle-shaped connection member can be completed by the extremely simple operation of inserting the needles into the cable and so this type of fitting is often used for coaxial cables.
If this method can be applied to the branching and connection of twisted pair cables, then it would be possible to simplify the connection work and to alleviate the problem of waveform distortion. However, this has not been realized for the following reason.
When a twisted pair cable is used as a communications path, it is necessary to connect the polarities of wires correctly since each of the wires are biased to mutually different polarities. However, since twisted pair cables have two wires twisted together, the position of each wires is not fixed. Accordingly, it is not possible to perform accurate connection of the polarities with the needle-shaped connecting member since the inside of the cable is not checked. Because of presence of this problem, the inefficient connection method shown in FIG. 2 has to be used.
In addition, even if the connection method of FIG. 2 is used, a polarity identification method such as having colors on each of the power lines of the cable has to be used as a measure to prevent the connection of different polarities and this increases the cost of the cable.
It is therefore difficult to connect conventional communications cables since there are polarities in wires, and there is the problem of deterioration of the transmission path characteristics.